There is an increasing demand on mobile wireless operators to provide voice and high-speed data services, and at the same time, these operators want to support more users per base station to reduce overall network costs and make the services affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities are needed. The available spectrum for wireless services is limited, and the prior attempts to increase traffic within a fixed bandwidth have increased interference in the system and degraded signal quality.
One problem exists when prior art omni-directional antennas are used at the base station because the transmission/reception of each user's signal becomes a source of interference to other users located in the same cell location on the network, making the overall system interference limited. Such an omni-directional antenna is shown in FIG. 1(a). In these traditional mobile cellular network systems, the base station has no information on the position of the mobile units within the cell and radiates the signal in all directions within the cell in order to provide radio coverage. This results in wasting power on transmissions when there are no mobile units to reach, in addition to causing interference for adjacent cells using the same frequency, so called co-channel cells. Likewise, in reception, the antenna receives signals coming from all directions including noise and interference.
An effective way to reduce this type of interference is to use multiple input-multiple output (MIMO) technology that supports multiple antennas at the transmitter and receiver. For a multiple antenna broadcast channel, such as the downlink on a cellular network, transmit/receive strategies have been developed to maximize the downlink throughput by splitting up the cell into multiple sectors and using sectorized antennas to simultaneously communicate with multiple users. Such sectorized antenna technology offers a significantly improved solution to reduce interference levels and improve the system capacity.
The sectorized antenna system is characterized by a centralized transmitter (cell site/tower) that simultaneously communicates with multiple receivers (user equipment, cell phone, etc.) that are involved in the communication session. With this technology, each user's signal is transmitted and received by the base station only in the direction of that particular user. This allows the system to significantly reduce the overall interference in the system. A sectorized antenna system, as shown in FIG. 1(b), consists of an array of antennas that direct different transmission/reception beams toward each user in the system or different directions in the cellular network based on the user's location.
The radiation pattern of the base station, both in transmission and reception, is adapted to each user to obtain highest gain in the direction of that user. By using sectorized antenna technology and by leveraging the spatial location of mobile units within the cell, communication techniques called space-division multiple access (SDMA) have been developed for enhancing performance. Space-Division Multiple Access (SDMA) techniques essentially creates multiple, uncorrelated spatial pipes transmitting simultaneously through beamforming and/or precoding, by which it is able to offer superior performance in multiple access radio communication systems.
This method of orthogonally directing transmissions and reception of signals is called beamforming, and it is made possible through advanced signal processing at the base station. In beamforming, each user's signal is multiplied with complex weights that adjust the magnitude and phase of the signal to and from each antenna. This causes the output from the array of sectorized antennas to form a transmit/receive beam in the desired direction and minimizes the output in other directions, which can be seen graphically in FIG. 2.
While known methods exist in the conventional multi-user multiple antenna systems that employ an orthogonal precoder to place weightings on the spatially orthogonal beamforming transmissions, the known methods and systems are not optimized in the precoding operations, and thereby fail to optimize the performance on the network. The present invention resolves these problems. Further, the installation of many antennas at single base stations can have many challenges which are resolved by the present invention. Since the available spectrum band will probably be limited while the requirement of data rate will continuously increase, the present invention also supports an expansion of the available spectrum over known methods for precoding in the cellular network.
The various components on the system may be called different names depending on the nomenclature used on any particular network configuration or communication system. For instance, “user equipment” encompasses PC's on a cabled network, as well as other types of equipment coupled by wireless connectivity directly to the cellular network as can be experienced by various makes and models of mobile terminals (“cell phones”) having various features and functionality, such as Internet access, e-mail, messaging services, and the like.
Further, the words “receiver” and “transmitter” may be referred to as “access point” (AP), “base station,” and “user” depending on which direction the communication is being transmitted and received. For example, an access point AP or a base station (eNodeB or eNB) is the transmitter and a user is the receiver for downlink environments, whereas an access point AP or a base station (eNodeB or eNB) is the receiver and a user is the transmitter for uplink environments. These terms (such as transmitter or receiver) are not meant to be restrictively defined, but could include various mobile communication units or transmission devices located on the network.